The disclosure relates generally to rotary steam dryers of the type which dry wet product material such as soybean meal, and more particularly to a control system for use in such a rotary steam dryer.
Dryers for drying wet soybean meal or the like are well known in the art. The most common form of dryer is a rotary steam tube dryer. In such dryers, steam is used as the heating medium. The dryers customarily include a dryer cylinder into which the wet product material to be dried is administered. Extending the length of the dryer cylinder are steam tubes which are arranged in concentric circles. The dryer cylinder and steam tubes are revolved as wet product material is fed continuously into one end of the dryer cylinder through a stationary throat piece so as to be heated by tumbling in direct contact with the steam tubes. The heating of the wet product material by the steam tubes causes the moisture within the wet product material to evaporate. A small air current passing through the dryer cylinder absorbs and carries off the evaporated moisture in the form of water vapor.
Normal operating steam pressure utilized in such systems falls within the range of 0 to 150 pounds per square inch. The drying temperature is definitely established by the steam pressure within the tubes. The amount of steam condensed within the tubes is directly proportional to the heat load. Under no load, very little steam is condensed, only that required for radiation and stack losses. When wet product material is introduced into the dryer cylinder, the rate of condensation of the steam within the steam tubes increases and continues to increase until the system reaches equilibrium.
Such dryers are constant temperature variable heat input devices if the steam pressure within the steam tubes is maintained constant. Thus, for a given heat load, the pressure of the steam and subsequently the temperature are determined by the parameters of the system. If the pressure is maintained constant, the system will supply the necessary steam to supply the demands of the heat load.
The steam is fed into the tubes through a proportioning steam valve controlled by a dryer control. One such dryer control is fully described and claimed in U.S. Pat. No. 3,778,707 which issued on Dec. 11, 1973 to the Assignee of the present invention. The dryer control described in that patent includes a moisture detector as the moisture measurement device. The moisture detector is at the discharge of the dryer and continuously senses the moisture content of the product material being discharged from the dryer. The moisture detector provides an electrical signal which is proportional to the product material mositure and is coupled back to a mositure display where the moisture is continuously displayed on a meter which reads out directly in percent moisture. The display of the system also scales the electrical signal and sends it to the controller portion of the system. The controller periodically compares the moisture signal to an operator selected moisture setpoint and makes corrections in the opening of the proportioning steam valve. The amount of valve opening depends upon whether the processed product material moisture content is above or below the setpoint. The sampling period of the controller is dependent on the through time of the dryer with the period somewhat longer than the through time.
While such rotary dryer systems have represented a great advancement in the art, there remains substantial room for improvement. Because the sampling or correction intervals are quite long, on the order of 30 minutes, changes in heat load to the dryer can cause substantial condensation of the steam and loss of steam pressure within the steam tubes so that the wet product material to be dryed is not sufficiently dried. Hence, a lag problem has resulted by virtue of the fact that the steam pressure within the steam tubes varies with changing heat loads during the sampling or correction intervals. Therefore, large material feed rate variations result in large deviations in output moisture because no corrections in steam pressure are made between correction intervals. In extreme cases, water-logging of the dryer has resulted by virtue of a greatly increased feed rate during the correction intervals.
It is therefore a general object of the present invention to provide a new and improved rotary steam dryer for drying wet product material such as soybean meal or the like.
It is a more particular object of the present invention to provide a new and improved control system for use in such a rotary steam dryer.
It is a more particular object of the present invention to provide a new and improved control system for use in a material product steam dryer which not only provides corrections in steam pressure at periodic intervals, but also, continuously senses the steam pressure and maintains the steam pressure substantially constant during the periodic intervals.